1. Field of Applicable Technology
The present invention relates to a flat configuration cathode ray tube for applications such as a color television receiver, computer display terminal, etc.
2. Prior Art Technology
In recent years, display units which are thin in shape have come to be increasingly utilized for display of images and characters. The flat configuration cathode ray tube is one type of such an apparatus, for example as described in Japanese patent laid-open Nos. 60-189848 and 60-193242.
FIG. 1 is an oblique view of a prior art flat configuration CRT, formed of electrodes which are contained within an external vacuum sealed container, e.g. a glass vacuum vessel, and FIG. 2 is a corresponding partial plan cross-sectional view. For clarity of description, most of this containing vessel has been omitted from the drawings. The horizontal display direction of images or characters displayed by the CRT is indicated by a horizontal arrow H, and the vertical display direction by a vertical arrow V. A plurality of mutually separate line cathodes 101, each extending in the vertical direction, are arrayed with a fixed pitch along the horizontal direction. The line cathodes 101 are each formed of tungsten wire having a surface coating of a cathode oxide. The number of line cathodes 101 and the pitch, where the term "pitch" as used herein signifies a value of regular spacing between linearly arrayed elements at which they are arrayed are optional. However assuming for example that the display image size is 10 inches, then the array pitch of the line cathodes 101 would be 10 mm, and 20 line cathodes would be used, each having a vertical height of 160 mm. An image display section 102 is separated from the line cathodes 101 by a predetermined spacing. Vertical scanning electrodes 103 are disposed behind the line cathodes 101. Each of the vertical scanning electrodes 103 is elongated in the horizontal direction, and the electrodes are arrayed with a fixed pitch in the vertical direction, and supported such as to be mutually electrically isolated upon a supporting member 104. In general, the number of the vertical scanning electrodes 103 is 1/n times the total number of horizontal scanning lines of the display, where n is an integer. However in this example it will be assumed that the number of these vertical scanning electrodes 103 is identical to the number of horizontal scanning lines (i.e. if the CRT is to be utilized for a usual television display, approximately 480 lines, for the NTSC standard). Between the line cathodes 101 and the image display section 102 are successively positioned, extending from the line cathodes 101, a set of first grid electrodes (hereinafter abbreviated to G1 electrodes) 105, a second grid electrode (hereinafter abbreviated to G2 electrode) 106, a third grid electrode (hereinafter abbreviated to G3 electrode) 107, and a fourth grid electrode (hereinafter abbreviated to G4 electrode) 108. The G1 electrodes 105 each are respectively identical and electrically separate flat-shaped electrodes having respective apertures 109 (shown in FIG. 2) formed therein, with the apertures being positioned in correspondence with respectively ones of the line cathodes 101 as indicated in FIG. 2. The G2 to G4 electrodes are each formed as a thin flat plate, with apertures formed therein. Respective video signals are applied to the G1 electrodes 105, for executing electron beam modulation. The G2 electrode 106 and the G3 electrode 107 have respective apertures 110 and 111 (shown in FIG. 2) formed therein which are positioned in correspondence with the apertures of the G1 electrodes 105, but are not divided in the vertical direction. The G4 electrode 108 has apertures 112 formed therein which may be identical to the apertures 110, 111 of the G2 electrode 106 and G3 electrode 107 respectively, or which may be of greater width in the horizontal direction than in the vertical direction as illustrated in FIG. 1. Between the G4 electrode 108 and the image display section 102 are disposed a set of horizontal deflection plates 113, having mutually opposing pairs of horizontal scanning electrodes 115A, 115B and 115C formed thereon as shown. Each of these pairs of horizontal scanning electrodes consists of vertically extending electrodes which are positioned symmetrically with respect to the axis of a corresponding electron beam which is emitted from the line cathodes 101 (as described in detail hereinafter). The centers of these pairs of horizontal scanning electrodes are spaced at regular intervals which are identical to the pitch of the line cathodes 101. The horizontal deflection electrodes 115A, 115B and 115C are formed by means such as etching of a metallic layer formed by metal plating or evaporative deposition upon surfaces of supporting members 114, each of which is formed of an electrically insulating material. As a result of voltages applied thereto, the horizontal deflection electrodes 115A, 115B and 115C execute horizontal electron beam focusing, electron beam horizontal deflection, and beam acceleration.
A photo-emissive layer consisting of a screen phosphor layer 116 and a metal back layer electrode 117 is formed on the inner surface of a portion of the glass containing vessel, to thereby constitue an image display section 102. The phosphor layer 116 is formed of red (R), green (G) and blue (B) stripes or dots arrayed successively along the horizontal direction, in the case of a color display.
The operation of this flat configuration CRT is as follows. Heating currents are caused to flow in the line cathodes 101 shown in FIG. 1, which are fixed at a common potential, while a potential that is more negative than that common potential is applied to all of the vertical scanning electrodes 103 other than a currently selected one of the vertical scanning electrodes 103. Respective electron beams, arrayed along a horizontal line corresponding to the selected one of the vertical scanning electrodes 103, are thereby emitted from the vertical scanning electrodes 103, towards the G1 electrodes 105 and the G2 electrode 106. A potential which is higher than that of the line cathodes 101 by approximately 100 to 500 V is applied to the G2 electrode 106, causing the electron beams to pass through respective ones of the apertures 110, 111 formed in the G1 electrodes 105 and G2 electrode 106 respectively, after having passed through the apertures 109 of the G1 electrodes 105. Control of the level of current of each electron beam is executed by varying the voltage which is applied to the corresponding one of the G1 electrodes 105. After passing through the apertures 110 of the G2 electrode 106, the electron beams pass through the apertures of the G3 electrode 107 and G4 electrode 108, then pass midway between respective ones of the pairs of mutually opposing sets of horizontal deflection electrodes 115A, 115B and 115C. Predetermined voltages are applied to the above electrodes, for causing the electron beams to form respective small spots on the phosphor layer of the image display section 102. Focussing of each beam in the vertical direction is implemented by a static electron lens which is formed at the exit from a corresponding one of the apertures 112 of the G4 electrode 108, while beam focusing in the horizontal direction is implemented by an electron lens formed by the horizontal deflection electrodes. Horizontal focus adjustment can be executed by variation of the center values of respective voltages which are applied between each of the opposing pairs of horizontal deflection electrodes 115A, 115B and 115C, The horizontal deflection electrodes 115A, 115B and 115C are mutually interconnected by respective pairs of common conductors 115A-a,b 115B-a,b and 115C-a,b. During each horizontal scanning interval, a sawtooth waveform deflection voltage or a staircase waveform deflection voltage is applied between each of these pairs of common conductors, superimposed upon the respective focus voltages. The respective electron beams are thereby deflected through a predetermined horizontal scanning width as they fall on the phosphor layer 116 of the image display section 102, to produce emission of light. In the case of a color display CRT, timing control of the modulation signals which are applied to the G1 electrodes 105 can be synchronized with timings at which the electron beams fall upon respective color stripe or dot portions of the phosphor layer 116 of the image display section 102 during each horizontal sweep.
Vertical scanning will be described referring to FIGS. 3 and 4. As stated above, control of electron emission from the line cathodes 101 is executed by selectively determining the voltages applied to respective ones of the vertical scanning electrodes 103. Specifically, the potential of the space surrounding a line cathode, adjacent to a specific one of the vertical scanning electrodes 103, is made positive or negative with respect to the potential of the line cathodes 101, in accordance with the voltage applied to that vertical scanning electrode. Electron beam switching for vertical beam scanning is thereby implemented. The smaller the spacing between the line cathodes 101 and the vertical scanning electrodes 103, the smaller will be the level of voltage that is required to control ON/OFF switching of the electron beams emitted from the line cathodes 101. If interlace scanning is used, then vertical scanning signals will be applied to the vertical scanning electrodes 103 during a first field interval (i.e. a first vertical scanning interval, designated as 1 V.sub.A in FIG. 4) such that a condition in which electron beams are generated (referred to in the following simply as the ON state) is produced during the first horizontal scanning interval (i.e. 1 H interval) at the start of the field interval by the vertical scanning electrode 103A, as illustrated in FIG. 4. During the next 1 H interval, a signal is applied to the vertical scanning electrode 103C to establish the electron beam ON state, and thereafter signals are successively applied to the remainder of the odd-numbered vertical scanning electrodes to successively establish the electron beam ON state during sequential 1 H intervals. This is terminated when vertical scanning electrode vertical scanning electrodes 103 at the bottom of the display is reached. During the succeeding field interval (indicated in FIG. 4 as 1 V.sub.B) signals to establish the electron beam ON condition are applied during respective 1 H intervals to the even-numbered vertical scanning electrodes, beginning with electrode 103B and terminating with electrode 103Y.
Referring to FIGS. 5 and 6, a description will be given of a signal processing system for supplying signals to the G1 electrodes of a flat configuration CRT such as that described above, having a plurality of electron beam sources arrayed along the horizontal direction, for the case of application to a television display. A timing pulse generator 144 generates timing pulses to be applied to drive circuits that are described hereinafter, in response to a television sync signal 142. A corresponding television video signal 141 is converted to successive digital data values by an analog/digital converter 143, and a set of these data values are sequentially inputted to a line memory 145 during a 1 H interval. When all of the set of data values for a 1 H interval have been supplied to the line memory 145, the data values are then simultaneously transferred to a second line memory 146, and during the succeeding 1 H interval a new set of digital data values are inputted to the line memory 145. The data values which have been transferred to the line memory 146 are held therein during a 1 H interval, and are transferred to a digital/analog converter (or pulse-width converter) 147, to be converted to corresponding analog signals (or pulse-width modulated signals). These are amplified, and applied to the G1 electrodes 105 of the CRT. The line memories are thereby used to perform time-axis conversion, as can be understood referring to FIG. 6. Designating the number of electron beams that are used to scan the display region (i.e. the number of line cathodes) as A, and the duration of a horizontal scanning interval (1 H) of the video signal as T, a portion of the input video signal 151 in FIG. 6 that occurs during an interval T within a 1 H interval is divided into A segments, each having a duration of T/A. The duration of these signal segments is then multiplied by the factor A, to thereby extend that duration to become equal to T. An example of a time-axis expanded signal segment is designated as 152 in FIG. 6. This process is executed for the entirity of each 1 H interval of the input video signal, and as a result of sequential scanning in the vertical direction by the scanning signals applied to the vertical scanning electrodes 103, a complete display image is produced.
In order to implement electron beam horizontal focusing and deflection by the horizontal deflection deflection plates 113 described above, deflection voltages having a periodic waveform such as a sawtooth waveform are applied to the deflection plates for executing horizontal deflection, together with DC voltages superimposed thereon for executing horizontal focusing. The levels of these DC voltages are approximately in the range 1 to 20 KV. Since the horizontal deflection electrodes 115A, 115B and 115C are formed directly upon the supporting members 114, e.g. by etching of a metallic layer formed on the supporting member surface, electrical discharge or insulation breakdown can readily occur in regions between mutually adjacent ones of the horizontal deflection electrodes 115A, 115B and 115C. Structures for such horizontal deflection plates have been proposed in the prior art for overcoming this problem, for example as described in Japanese patent laid-open No. 62-58554.
Such horizontal deflection plates will be described referring to FIGS. 7 and 8, which are respective oblique views of two examples of a pair of such prior art deflection plates. Each deflection plate consists of a supporting member 118 formed of an electrically insulating material, with three horizontal deflection electrodes 119a, 119b and 119c mounted thereon (only the electrodes mounted upon one face of each supporting member being shown, for simplicity of description). In the example of FIG. 7, the horizontal deflection electrodes 119a and 119c are formed directly upon a surface of a supporting member 118, as in the preceding prior art example, e.g. by etching of a metallic layer formed by a process such as evaporative deposition, while a centrally situated horizontal deflection electrode 119b is raised outward from the surface of the supporting member 118 by being mounted on a spacer which is attached to that surface, e.g. attached by glass frit. The horizontal deflection electrode 119b may be formed of metal plate. In the example of FIG. 8, each of the supporting members 118 has a horizontal deflection electrode 121c formed on a surface thereof in the same way as for electrodes 119c, 119a in FIG. 7. However the remaining two electrodes 121a and 121b are separated from the supporting member surface by different heights, by being respectively mounted on spacers 123, 122 which are of different width as measured perpendicular to the supporting member surface. Each of the horizontal deflection electrodes 121a (or 119a in FIG. 7) is situated at the low-voltage end of the deflection plates, i.e. is subjected to a relatively low DC focusing voltage, while the horizontal deflection electrodes 121c (or 121c in FIG. 8) are situated at the high-voltage end of the deflection plates, and the horizontal deflection electrodes 121b (or 119b in FIG. 7) are positioned intermediate between the high and low voltage ends of the deflection plates.
Although some improvement with respect to electrical discharge or insulation breakdown occurring between adjacent horizontal deflection electrodes is provided by these prior art examples when applied to a flat configuration CRT, by comparison with the simple horizontal deflection electrode configuration of FIGS. 1 and 2, the degree of improvement is not sufficient to enable satisfactory levels of horizontal focus voltages to be applied to the horizontal deflection plates of a very thin and compact flat configuration CRT. These problems of electrical discharge or insulation breakdown are made more severe as a result of electrical charge buildup which can occur in regions of the surface of each supporting member 118 between adjacent ones of the horizontal deflection electrodes, or the surfaces of the spacers, as a result of the electron beams passing between the electrodes or due to emission of secondary electrons from the image display section.
In addition, both with a prior art flat configuration CRT having horizontal deflection plates as shown in FIG. 1 and a CRT utilizing horizontal deflection plates as shown in FIGS. 7 or 8, there is a basic problem with respect to ensuring a satisfactory degree of vertical focus. Specifically, electron beam focussing in the vertical direction is mainly executed immediately after each beams exits from a corresponding one of the small apertures formed in the G4 electrode 108, i.e. respective electron lenses are formed. However each of these has a short focal length and a small depth of focus. Thus, the longer the distance which must be traversed after passing through such a vertical focus lens formed by the G4 electrode 108 until the image display section 102 is reached, the poorer will be the sharpness of vertical focus. It is therefore necessary to restrict the distance from the G4 electrode 108 to the image display section 102 to a sufficiently small value to obtain sharpness of vertical focus, (i.e. a small scanning spot width in the vertical direction). However in the prior art, that distance has not been determined on the basis of a requirement for sharpness of vertical focus, since a reduction of the distance results in a lowering of horizontal deflection sensitivity.